(Reuters) - The French-German consortium Areva-Siemens, the supplier of Finland's much-delayed Olkiluoto-3 nuclear reactor, has increased its claim against Finnish utility Teollisuuden Voima (TVO), TVO said late on Friday.

TVO and Areva have traded accusations about who is to blame for delays and extra costs, and the International Chamber of Commerce's (ICC) arbitration court is processing a dispute on cost overruns between the two sides.

Areva-Siemens in September said the start date of the reactor, which is planned to be Finland's fifth and biggest nuclear unit, will be pushed back to late 2018 - almost a decade later than originally planned.

TVO on Friday said Areva-Siemens raised its claim against it to 3.5 billion euros ($4.4 billion) from the previous 2.6 billion euros at the ICC arbitration.

TVO, whose owners include Finnish paper companies UPM and Stora Enso as well as utility Fortum, this week raised its own claim to 2.3 billion euros from an earlier 1.8 billion euros.

"The plant supplier committed to deliver the nuclear power plant unit by an agreed date according to the contract and the supplier is responsible for the delay and the ensuing costs," TVO executive Risto Siilos said in a statement.

The delay at the site in the west of Finland has put TVO's plan to build its next reactor, Olkiluoto 4, in doubt as Finnish government recently rejected TVO's application to extend its permits.